The invention relates to a bone spreader for spreading bone parts apart including two parallel tubular pin holders, which are connected to one another by a parallel guide system, and two pins to be connected to the bone parts that are to be spread apart. In connection with the invention, this term also includes screws. The pins are introduced parallel to one another into the bone parts that are to be spread apart. Their free sections are introduced into the pin holders. When these are now moved away from one another or moved closer to one another by means of the parallel guide system, this movement is transmitted to the bone parts. This type of spreader is especially suitable for distraction of two cervical vertebral bodies for the purpose of implantation of a cervical intervertebral prosthesis, as the vertebral bodies are guided parallel to one another during the distraction. However, this parallel attribute applies only with respect to the direction of the pin holders. Two degrees of freedom remain. These are, on the one hand, a rotation of the bone parts about the pin axis, which for various reasons is of no consequence in normal circumstances, and, on the other hand, a displacement in the direction of the pin holders, which displacement can be prevented by a locking device. For this purpose, a first known design of this locking device, disclosed in WO03/024344, uses a friction clamp, which in many cases is not secure enough. A second known design, disclosed in U.S. Pat. No. 6,340,363, uses a clamping screw or some kind of clamp. A clamping screw, however, cannot be maneuvered, or may be maneuvered only with difficulty, deep within the operating site. The issue remains, furthermore, of how a clamp can be designed so that it is both secure and easy to operate.